Radio transmitters and receivers are the primary points of energy consumption in wireless devices. The power supplies in wireless devices, such as batteries or energy-harvesting modules, will have a longer life if the communication protocols used by the radio are designed to minimize use of the receiver and transmitter circuits. The communication protocols are used to form network links and to negotiate end-to-end connections between devices. By keeping both transmitter and receiver in an OFF or sleep mode for a large percentage of time, the device will use less energy.
Transmission-initiated duty-cycling is a technique in which a transmitter sends out short strobe packets when the device has data packets that are ready to transmit. The strobe packets are sent intermittently and are triggered when the data packets are generated. The receiver, which is in an OFF or sleep mode most of the time, wakes up (i.e. enters an “ON mode”) at the end of every duty-cycle and listens on an assigned wireless channel. If the receiver senses a strobe packet from a transmitter, the receiver recognizes that the transmitter intends to communicate a data packet and, the receiver will stay in an ON mode to receive the data packet. Otherwise, if the receiver does not sense a strobe packet, then the receiver returns to an OFF mode to conserve power. Examples of transmission-initiated duty-cycled medium-access control (MAC) protocols that are designed for small-area networks are the Coordinated Sampled Listening (CSL) mode of the IEEE 802.15.4e protocol and X-MAC protocol.
Large networks of wireless devices, such as sensor nodes or motes, may communicate using CSL. Such sensor networks often operate unattended for months or years. Therefore, battery power in each device must be conserved for the devices to last for long periods.